Switched Reluctance Motor
The switched reluctance (SR) motor operates on the same basic principle as a variable reluctance stepper motor (Art. 39.4).
(a) Construction
Unlike a conventional synchronous motor, both the rotor and stator of a SR motor have salient poles as shown in Fig. 39.22. This doubly-salient arrangement is very effective for electromagnetic energy conversion.
The stator carries coils on each pole, the coils on opposite poles being connected in series. The eight stator coils shown in Figure are grouped to form four phases which are independently energized from a four-phase converter. The laminated rotor has no windings or magnets and is, therefore cheap to manufacture and extremely robust. The motor shown in Fig. 39.22 has eight stator poles and six rotor poles which is a widely-used arrangement although other pole combinations (like 6/4 poles) are used to suit different applications.
(b) Working
Usual arrangement is to energize stator coils sequentially with a single pulse of current at high speed. However, at starting and low speed, a
current-chopper type control is used to limit the coil current.
The motor rotates in the anticlockwise direction when the stator phases are energized in the sequence 1, 2, 3, 4 and in clockwise direction when energized in the sequence 1, 4, 3, 2. When the stator coils are energized, the nearest pair of rotor poles is pulled into alignment with the appropriate stator poles by reluctance torque.
Closed-loop control is essential to optimize the switching angles of the applied coil voltages. The stator phases are switched by signals derived from a shaft-mounted rotor position detectors such as Hall-effect devices or optical sensors Fig. (39.23). This causes the behaviour of the SR motor to resemble that of a dc motor.
(c) Advantages and Disadvantages
Although the newest arrival on the drives scene, the SR motor offers the following advantages:
(i) higher efficiency (ii) more power per unit weight and volume (iii) very robust because rotor has no windings or slip rings (iv) can run at very high speed (upto 30,000 rpm) in hazardous atmospheres (v) has versatile and flexible drive features and (vi) four-quadrant operation is possible with appropriate drive circuitry.
However, the drawbacks are that it is (i) relatively unproven (ii) noisy and (iii) not well-suited for smooth torque production.
(d) Applications
Even though the SR technology is still in its infancy, it has been successfully applied to a wide range of applications such as (i) general purpose industrial drives (ii) traction (iii) domestic appliances like food processors, vacuum cleaners and washing machines etc., and (iv) office and business equipment.
Comparison between VR Stepper Motor and SR Motor
